For as long as I have been in model railroading, I have never fully understood the out-puts listed on the back of the power packs. If I am looking to provide more DC power to the tracks, do I consider the TOTAL out-put? For example, the MRC Sound & Power lists 60VA TOTAL while the MRC Tech7 Ampac lists 20VA TOTAL, but both list close to the same VDC, 20 and 23. Would both packs give me close to the same track power?
The speed a train moves at is determined by the voltage. The DC to the track is variable usually from 0 to about 15 volts. Nearly all HO train locomotives are rated at 12 volts. The more voltage applied makes the motor turn faster.
VA is volts times amps, which is also Wattage, or very similar. This is power handling capacity. Lionel always listed power pack output in watts. HO power packs used to be listed in maximum amperage at 12 volts, usually around 1 or 2 amps. More recent packs list power output in VA.
You cannot control VA or Wattage. If your motor requires .5 amps at 12 volts, VA (or watts) is 6. If your motor requires .75 amps at 12 volts, then VA will be 9. Less efficient motors will require more VA.
VA, Amps, or wattage, represent the total amout of power a power pack can provide, this is different from voltage with is the force that pushes the electrons.
If your pack has enough power to run your locomotives, that is all that matters.
If you need 15 VA total, it does not matter if your power pack is capable of 25 VA or 100 VA, it will be within its operational capability.
-Kevin
Living the dream.
Thanks for your reply. I understand that HO locos run on variable DC from the power pack. What I don't understand is the following example. The same loco, running on the same section of track, and my multi-meter reading 15 volts from two or even three differnt packs, and the engine runs at different speeds. Something must be different from each pack since they all all reading the same DC voltage.
leewal The same loco, running on the same section of track, and my multi-meter reading 15 volts from two or even three differnt packs, and the engine runs at different speeds.
That makes no sense to me.
I hope someone with a better understanding of what you are seeing adds more comments.
Me too. Do you have a web site of your railroad?
Walt
leewal For as long as I have been in model railroading, I have never fully understood the out-puts listed on the back of the power packs. If I am looking to provide more DC power to the tracks, do I consider the TOTAL out-put? For example, the MRC Sound & Power lists 60VA TOTAL while the MRC Tech7 Ampac lists 20VA TOTAL, but both list close to the same VDC, 20 and 23. Would both packs give me close to the same track power?
As Kevin said, watts (DC power) = voltage X current. So the one unit will supply 60/12 or 5 amps and the other will supply 20/23 or a little less than amp.
So my opinion would be that the 60 watt unit will be more "powerful" than the 20 watt unit.
I believe that if the loco you are using for testing requires a lot of current to get to full speed the, higher wattage unit will have more current available.
(of course I may be totally wet in which case this post will soon short out.)
leewalhe same loco, running on the same section of track, and my multi-meter reading 15 volts from two or even three differnt packs, and the engine runs at different speeds.
is that 15V while the loco is running?
unless regulated, the voltage output from a DC source may drop depending on the current being supplied. this is due to the internal resistance of the power source. this is why the no-load output voltage is often higher than the rated voltage at the rated current.
Linn Westcott's articles in 1962 described a DC throttle that regulated the output voltage so that it was maintained as the loco drew more and less current
greg - Philadelphia & Reading / Reading
The transformer regulates the voltage. The efficiency of the transformer will regulate the amperage output. In basic terms. The speed of the train is dependent of the amperage (power) of the transformer. To make it seem simple. Think of voltage as the level of an unobstructed river and amperage as the flow of said river. Without changing the depth you increase flow. The increased flow increases the power.
I know electrical engineers hate the water analogy, but it gives a basic understanding of a difficult understanding of electrical flow.
Pete.
wrench567The transformer regulates the voltage.
voltage regulation means the voltage doesn't vary with the current being drawn.
the ratio of the transformer windings proportionally drops/increases the AC voltage on the secondary winding. the windings have resistance, hence the output voltage varies somewhat with the current being drawn
wrench567The speed of the train is dependent of the amperage (power) of the transformer.
presumably you mean the power delivered from the transformer thru the control circuit. a lotta power is wasted in the rheostats of older DC power packs used to drop the voltage to the loco.
PWM is much more efficient because it's either On/Off and when On, mosfet resistance is typically in the milliOhms
while the speed of locomotive depends on the current being drawn, that current is controlled by the the voltage being supplied.
but even if the voltage or current supplied to the motor is constant, the speed will vary as the mechanical load on the motor changes.
the BEMF depends on the speed of the motor regardless of load. it is actually generated by the motor and opposes the voltage supplied to the motor, limiting the current that will flow thru the motor
the BEMF drops when a train slows going up a grade which allows more current to flow into the motor because the difference between the supplied voltage and BEMF increases. but this increase in speed is not enough to maintain the speed.
the current thru the motor depends on the net voltage and winding resistance and will be higher if the supplied voltage is increased to maintain a constant BEMF
the voltage supplied to the motor can be adjusted to maintain a constant BEMF and speed as the mechanical load changes.
wrench567 I know electrical engineers hate the water analogy
the water analogy is fine. the flow depends on the pressure and is inversely dependent on restriction, the size of the pipe. heat flow is another example
Thanks Greg.
I was just trying to over simplify for a basic understanding for the OP. Many people don't know the difference between voltage and amperage. It took me a little time to understand how the coil in my old Barracuda could take 12 volts and bump it to 30,000 volts after being dropped down to 9 volts from the ballast resistor. Even though it distributed 30,000 volts the amperage was very low. Enough to bang your head on the hood but not enough to throw you across the street. Just enough to jump a .35 gap.
wrench567how the coil in my old Barracuda could take 12 volts and bump it to 30,000 volts after being dropped down to 9 volts from the ballast resistor.
the principle behind an automotive coil generating a large voltage spike is a bit different than a transformer.
somewhat similar to a capacitor holding accumulating charge resulting in a voltage, an inductor (coil) maintains the current passing thru it.
when the circuit passing a current thru an inductor is broken by the points/transistor in an ignition system, the current will seek a path resulting in whatever voltage is necessary. the gap in the spark plug provides that path. the voltage is whatever is needed to cross the gap, not that it matters, the temperature of the spark just needs to be sufficient to ignite the fuel
but this same principle is why flyback diodes are across each transistor in a the PWM h-bridge circuit of a decoder or PWM throttle. they provide a low voltage path for the current thru the motor windings to dissipate
leewalDo you have a web site of your railroad?
If this was for me, no I do not.
I will resurrect my YouTube channel when construction FINALLY begins.
My guess as I dont know how large a layout you have.
Your current is dropping the farther you get away from your track connection, your meter will measure 15 volts everywhere because your meter isnt putting the same load on the circuit as your loco will. Try adding more feeders to the areas where you experience the slow downs. Your power pack is fine as is.
"The true sign of intelligence is not knowledge but imagination."-Albert Einstein
http://gearedsteam.blogspot.com/
For example, the MRC Sound & Power lists 60VA TOTAL while the MRC Tech7 Ampac lists 20VA TOTAL,
The 60-Va has more 'power'.
but both list close to the same VDC, 20 and 23. Would both packs give me close to the same track power?
Think of it like body builders. One newby versus Arnold Shwartzenegger. Both have muscles (the voltage) but Arny has way more ability in his muscles (he has more amperage, 'oooomph' - grunt!).
Translated to model trains - you can run 3 locos at once instead of one.
Something must be different from each pack since they all all reading the same DC voltage.
Same as cars for example, two of same make, model, color etc etc will operate completely differently. Just the inherent nature of machines.
If your looking to raise the 'power' without changing 'voltage', look for a higher 'amperage' ("amp", "ampere", "a") rating on the transformer specs.
Just remember to be SAFE around electrical!
PMR
One of the better recent threads on the forum. Helpful and informative.
Rich
Alton Junction
Geared SteamMy guess as I dont know how large a layout you have. Your current is dropping the farther you get away from your track connection, your meter will measure 15 volts everywhere.
Great point, and questions for the OP...
1) Are you measuring voltage at the power pack output, or at the rails where the locomotives are running at different speeds.
2) How far apart are your feeders?
Two questions for the OP:
For the first question, if you're measuring with the same engine moving on the same track and getting the same voltage reading, it doesn't make sense that the engine would be moving at different speeds.
For the second question, if you're measuring max voltage and then comparing the engine speed at non-max throttle, I would expect it to behave differently since each power pack has different electronics.
Edit: Actually, for the second question, even at max throttle the voltage will likely be different with a load than if you measured with no load. Non-max throttle positions can make the speed difference more noticeable.
gregcis that 15V while the loco is running?
leewalThe same loco, running on the same section of track, and my multi-meter reading 15 volts from two or even three differnt packs, and the engine runs at different speeds. Something must be different from each pack since they all all reading the same DC voltage.
Which specific power packs are you using? Some use PWM which would output a square wave DC instead of solid DC voltage. Depending on how you are measuring the voltage, you could get quite different values from what's actually happening.
AlienKingWhich specific power packs are you using? Some use PWM which would output a square wave DC instead of solid DC voltage.
Great point, it might also help knowing what type of a meter is being used.
Jim
This is, I think, substantially preferable to what I would have suggested: building a megger with a high effective load resistance. A moral being that when you measure track voltage, have a train running in the block; the expanded version being to have the heaviest or highest-powered train you operate running while drawing current from the block.
I was going to suggest using some appropriate reference load resistor across the rails when measuring the voltage, but using trains for the load is more accurate as well as conceptually more elegant...
Soo Line fanI used a Fluke 87V true RMS meter connected to the rail.
Nice meter!
Thanks Kevin!
He said he had an Mrd sound and power. And an Mrc tech 7. As his power supplies. My question, is the track we are running on sectional? That does a good job of lowering power. Those joints acting as resistors to an extent. He just might need more feeder connections not more dc power. He didn't say anything about the track or configuration. That could be the whole problem to his varying powershane
A pessimist sees a dark tunnel
An optimist sees the light at the end of the tunnel
A realist sees a frieght train
An engineer sees three idiots standing on the tracks stairing blankly in space